UCP1 and hyperinsulinemic hypoglycemia, familial, 4: In skeletal muscle, besides its induction by insulin and exercise [40,48,49], FGF21 is also produced in mitochondrial myopathies [50,51] and it has been proposed as a putative serum biomarker for diagnostic use [52], but also in impaired mitochondrial fatty acid oxidation [53], muscle-specific autophagy deficiency [54], and transgenic overexpression of Akt1, perilipin-5 [55], or uncoupling protein 1 (UCP1) [56].